This invention relates generally to radar signal processing and in particular to an apparatus and method for processing radar return data for a weather radar system.
Pulsed doppler radars that are required to process radar return signals with a large dynamic range such as in weather radar applications often use a feed-forward closed-loop automatic gain control (AGC) to maintain the amplitude of the input signals within the dynamic range of the radar's receiver and in particular within the dynamic range of the receiver's analog to digital (A/D) converter(s). The AGC may be controlled by analog control before the A/D converter or by digital control after the A/D converter. The radar return signals typically are integrated in a sliding window, block averager or exponential integrator by analog or digital means in order to adjust a linear automatic gain controlled amplifier or attenuator. An AGC signal in multiples or sub-multiples of a decibel (dB) is fed forward, scaled and added to a logarithm of the fixed point A/D data to produce the logarithm of the radar return signals. Radar jamming and pulse interference from other radars result in limiting in the receiver's A/D converter and generally control the AGC signal thereby reducing the radar's dynamic range and ability to suppress ground clutter.
Another known approach in the art is to delay the radar return signals until the magnitude, power or log of the return signals set the gain of a linear amplifier or attenuator which results in the delayed return signals being within the dynamic range of the A/D converter(s) in the radar receiver. However, analog and video delays are difficult and expensive to control accurately over a power supply tolerance and operating temperature range for the processing required to control the gain of the linear amplifier or attenuator.
Pulse interference detectors are employed in radar systems to eliminate the processing of random impulse type interferences. One known approach in the prior art examines range cells adjacent to range cell M, i.e. M-1 and M+1, while processing a range M return signal to detect a pulse interference but does not generate replacement data for an interfered-with pulse at range M.
Another approach known as coincidence technique for random pulse interference uses a sweep to sweep coincidence criteria which requires an amplitude threshold to be exceeded on two (or more) successive sweeps at the same range before a radar return is accepted as a valid signal. However, this approach causes a loss in detection sensitivity because successive sweeps of weak signals may not reach the required amplitude threshold.
Another approach known in the prior art as an amplitude difference interference rejector uses a sweep to sweep amplitude comparison for each range cell. A random interference return with a large amplitude has a low probability of occurring in adjacent sweeps at the same range. By comparing the amplitude difference between the present sweep and previous sweep, random pulse interference can be rejected without appreciably affecting strong valid radar returns which are correlated within the antenna beamwidth. However, in a typical coherent radar system when a pulse interference is detected, eight to ten radar return signals being processed end up being lost along with the radar return for the range cell tested.